Recently, interests on energy storage technologies are more increased. As the energy storage technologies are extended to cellular phones, camcorders and notebook PC, and further to electric vehicles, the demand for high-energy concentration of a battery used as a power source of such an electronic device is increased. A lithium ion secondary battery is one of the most satisfactory batteries, and many studies are now in active progress.
Among secondary batteries currently used, a lithium secondary battery developed in the early 1990's includes an anode made of carbon material capable of intercalating or disintercalating lithium ions, a cathode made of lithium-containing oxide, and a non-aqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a mixed organic solvent.
The lithium secondary battery has an average discharge voltage of about 3.6 to 3.7V, which is advantageously higher than those of other batteries such as alkali batteries or nickel-cadmium batteries. To give such a high operation voltage, an electrolyte composition electrochemically stable in a charging/discharging voltage range from 0 to 4.5V is required. For this purpose, a mixed solvent in which a cyclic carbonate compound such as ethylene carbonate or propylene carbonate and a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate are suitably mixed is used as a solvent of electrolyte. A solute of electrolyte commonly uses a lithium salt such as LiPF6, LiBF4 and LiClO4, which acts as a source for supplying lithium ions in a battery and thus enables the lithium battery to operate.
Meanwhile, to improve life cycle and performance of a battery, non-aqueous electrolyte solutions obtained by various compounds to the above-mentioned non-aqueous electrolyte solutions have been proposed. For example, a solvent with a fluoro group such as fluoroethylene carbonate (FEC) is known as enhancing ion conductivity and thus improving life cycle and performance of a battery when being added to a non-aqueous electrolyte solution since it has a high polarity and an increased oxidation potential. However, a battery having a non-aqueous electrolyte solution to which a solvent with a fluoro group is added exhibits a great increase of thickness when the battery is kept at a high temperature. Namely, the battery is swelled due to gas generation, so problems occur in sets such as cellular phones and notebooks.
Meanwhile, Korean Laid-open Patent Publication No. 2003-59729, Japanese Laid-open Patent Publication No. 2003-323915, No. 2002-134169 and No. 2003-173816 disclose non-aqueous electrolyte solutions containing siloxane compounds. However, these documents fail to teach the addition of a solvent with a fluoro group or any effects obtained by the addition.